A bit of a damp squib?

Rainwater harvesting is a good idea, but potential users need to consider all the implications, says Steve Cupples.

Rainwater harvesting has become the "next best thing" when it comes to saving our valuable water supplies. However, when you look carefully at the projects where rainwater harvesting has been introduced it is rarely used beyond that which is necessary for running washing machines, flushing toilets and watering the garden.
In real terms, the cost of collecting enough rainwater to be effectual and to make it safe is a huge investment. The question of contamination makes the use of rainwater complicated.
Rainwater contamination can arise from several obvious sources:
·The roof itself carries dust and dirt from the local atmosphere
·The rainwater will collect atmospheric pollutants while falling, hence acid rain (although this is less of a problem today)
·Bird droppings and feathers as well as the occasional carrion all add to the potential problem as well the debris that is left.

All of these are not an issue if the water is to be used on irrigation, but if you wanted to use it for the preparation and cooking of food products or for washing down floors or surfaces, the degree of filtration and treatment required needs to be of superior standard.
The ability to harvest rain in reality has little impact within industry due largely to two issues - the ability to harvest effectively, and to store and make safe for use.
So rainwater collection is not as easy as it may first appear.
A large roof is the best solution for easy collection, but gathering all the water from any roof to a single point is not always that simple.
Positive action
Potential users of rainwater must consider how they intend to use it. It is assumed that rainwater is clean. This is far from the truth as the water has significant bacterial activity and when stored for any length of time the bacteria will almost certainly proliferate unless positive action is taken to prevent growth.
Rainwater harvesting is also dependent on rainfall and, at its best, rain is unpredictable. So to accommodate these fluctuations it would be necessary to plan on a large size tank to cover the dry times as much as possible, by effectively collecting all the rain available at any given time.
In terms of cost savings, if you work with say an average of 5cm rain fall per month this would give 780m3/annum or 780,000l.
If a business was using 6m3 of water an hour for wash down purposes. Based on calculations of harvested rain, and assuming it was an average year for rain and all rainwater could be collected, an organisation would get only 50% of its washing requirement.
On this basis it would therefore only save £700 per year.
The harvesting of rainwater can be effective and can offer significant savings, but potential operators must consider all of the implications.
Here are a few pointers to consider:
·If you have a large collection area, like a roof, then this is a positive for the project. However, do not forget to give consideration on how it can be collected and delivered to a single point for treatment and processing. Most roofs evacuate rainwater through several downpipes located around the building perimeter, which makes collection to one point challenging. Adding pumps at various collection points will not necessarily solve this problem as there will be no reduction in carbon omissions
·You may consider intercepting your groundwater collection as well as your roof discharge. If your groundwater evacuation lends itself to this then also consider what else you may be collecting other than rainwater, for example: petrol; oil; and diesel spillage from car parks; dust; dirt; and other debris from hard standing areas all of which may have adverse effects on water quality
·Consider the application of the rainwater. If it is for a specific process, what quality of water will be required for the process, as this will dictate the type of treatment and the capital cost of the project? If you have a large number of staff, can this be used for lower quality applications such as toilet flushing or vehicle wash water? This will minimise the processing costs and maximise the saving
·Consider the storage of water. Rain is at best erratic and therefore you need to consider what capacity you can accommodate on site. What would be the implications if large storage tanks are to be sited outside? Can they be buried underground? Do you need planning approval? How convenient is its location to the collection and to the potential use of the water?
·Consider your current use of water. Can it be re-used? Reusing water only once will reduce incoming water bills by 50%, and this is also the same for effluent charges. This is often the most cost effective solution in water use minimisation
·Safety in collecting and using rainwater. The operator takes the full legal responsibility for its quality and safe use, so you would need to undertake a full risk assessment which would include, for example:
·What contamination will I have to deal with: asbestos sheet roof; coated roof; slate roof; felt roof; and what can leach into the water from the roof construction?
·Water contains living organisms. Is this an issue for what I want to use the water for and, if so, how will I contain bacterial growth in warm weather?
·Will I need to circulate water all of the time to minimise the risk of water becoming stagnant? If so, what pumping costs are involved?
·How clean does my process require the water to be? Potable or just clear and free from solids?
·What equipment do I need to achieve this quality?
·What additional costs are involved, capital purchase and operational?
·How can I check that the quality target is being achieved, and how can this be safeguarded?
Steve Cupples is managing director of Industrial Purification Systems. www.industrial-purification.co.uk.